If you have lots of time, then consider going via Seoul or Tokyo as the other carrier's products can be better. However, I did fly UA this month in economy plus to Beijing and it wasn't terrible. The AVOD is on demand and the food is ok but it was a non-stop.
Those extra hours in another airport could be better spent in your hotel in Shanghai. Also, UA has a decent arrival schedule into PVG. I've flown via SFO, ORD and EWR to PVG and they get you in the middle of the afternoon. I've thought about routing to LA to get the 787 but that adds so many hours to the flight that even in J, it isn't worth it to me.
I haven't flown in CX economy, only upfront. Even in J, doesn't make sense unless you want AA miles (although you need to check the fare as CX is stingy) as it is a longer flight to HK and then to go back north.